High frequency amplifier



June 2, 1931. P. J. TOWNSEND 1,808,150

v HIGH FREQUENCY AMPLIFIER Filed Nov. 1, 1927 l l l l l l l flerc/m/ J7557/75 end gvmi V ATTOEA EY Patented June 2, 1931 PATENT oFFicsPERCIVAL J. TOWNSEND, OF LOS ANG-ELES, CALIFORNIA HIGH FREQUENCYAMPLIFIER Application filed November 1, 1927. Serial No. 230,311.

This invention relates to high frequency currents, and especially to theamplification of radio frequency signals carried by such currents.

The amplifier usually used for such purposes is an electronic emissionamplifier, consisting of a sealed evacuated vessel in which there areseveral electrodes. One of the electrodes serves to emit electrons, andis usually in the form of a filament arranged to be heated by thepassage of a current therethrough. Another electrode is an anode orplate to which the electrons travel, forming a space current, and whichis maintained at a potential positive with respect to the filament bythe aid of an external circuit connecting these electrodes. There isalso a control electrode usually in the form of a grid, which isinterposed in the field between the filament and plate. It has beenfoundthat even minute changes in the potential difference between thefilament and grid cause comparatively large changes in the spacecurrent.

Therefore to secure amplification, it is necessary to subject the gridand filament to potential differences correspondingto the impulses to beamplified; and there result comparatively large variations in currentfiow in the circuit connecting the filament and plate. These variationsin turn can be reamplified in a succeeding stage. 7

Futhermore, such a vacuum tube can also serve as a detector for themodulations carried by the radio frequency current, by the addition of asmall series condenser in the grid-filament circuit and adjacent thegrid. Both these modes of operation of vacuum tubes are well-understoodat this time, and need no further elucidation.

It is also common to secure a very large degree of amplification fromonetube by passing back a portion of the amplified impulses to the inputside, as by coupling the input and output circuits. It is one of theobjects of my invention to provide a novel and efficient form offeed-back to secure these results.

In such regenerative systems, difficulty has I been experienced in thepast to control the w feed-back so as to keep the system stable and asbattery to prevent it from setting up self-oscillations.

It is another object of my invention to pro- .vide a novel and simpleform of control for gradually froma reversed feed-back to regenerativecondition as the broadcast range is traversed.

My invention possesses many other advantages, and has other objectswhich may be made more easily apparent from a consideration of severalembodiments of my invention.

For this purpose, I have shown a few forms in the drawings accompanyingand forming part of the present specification. I shall now proceed todescribe these forms in detail,

which illustrates the general principles of my invention; but it is tobe understood that this detailed description is not to be takenin alimiting sense, since the scope of my invention is best defined by theappended claims.

Referring to the drawings; 7

Figure 1 is a wiring diagram of a radio system embodying my invention;and

fFig. 2 is a diagram of a modification there- 0 In Fig. 1, I show athermionic amplifier and detector 11, having a filament 12, anode 13,and control electrode or grid 14, all operating as hereinbeforesetforth. The filament is shown as arranged to be heated by. current from asource of electric energy, such The filament 12 and grid 14 aresubjected to potential differences corresponding to signal impulses thatexist in circuit 16-17;

this is accomplished by providing a coil 18 in circuit 16-17 that isinductively coupled to another coil 19. One terminal of this latterco-il connects directly to the filament 12; and the other terminalconnects to grid 14: through the usual grid condenser 20. In

necting electrode 13 to filament 12.

- the input side.

order that the system be selective to a definite frequency, a variablecondenser 21 is provided, bridging coil 19, and forming with it atunable circuit. It is now well-understood that the potentialdifierences existing across the terminals of the coil 19 or thecondenser 21 are a maximum for a definite frequency dependent upon thesetting of condenser 21. Thus by tuning this circuit, signals of adefinite frequency can be accentuated or selected from the rest. Circuit16-17 can connect to the usual pickup circuit or to the output of anamplifier interposed between such a pick-up structure and the detectorstage.

These potential differences existing across this turnable circuit causecorresponding large variations in the space current, and in the currentflowing inan external circuit con- This circuit includes plate 13, coil2-2, translating device 23 such as a phone or loud speaker or couplingcoil, battery 24, and filament 12.

The system can be supplemented as is well understood, by one or morestages of audio frequency amplifiers.

Coil 22 forms one of the elementsfo-r a feed-back circuit and is shownas closely coupled to the coils 18 and 19. It is also closely coupled toa coil 25 that is connected to a variable condenser 26 which inturnconnects to the filament 12. Upon proper direction of winding ofcoil 25, a large potential difference is induced across both coils22-and 25 in series which is effective to transfer-energy to This largepotential difference is secured 'due to the transformer effect of coil22 on 25. Coil22 carries theoutput current and has apotential differencedue .to

the flow of this current; and coil 25 has a much larger potentialdifference because of the larger number of turns therein. The variablecondenser 26 serves to control the degree of regeneration in a mannernow to be explained. a

It is to be noted that coil 25 is adjacent coil 19, and coil 22 isspaced therefrom. Thus when condenser 26 is set at-a low value, thecircuit of coil 25 has a high impedance; it is therefore of littleeffect to serve as a. coupling coil between the input and the-outputcircuits, the condition approaching an open circuit between the upperterminal of coil 25 and filament 12. Since coil 22 isspaced from coil 19the effect is that there is little transfer of energy between the inputand output circuits. However, as condenser 26 is varied to increase thecapacity, coil 25 becomes more paths in the output circuit. Thus coil 22is directly in series with the translation device 23; and coil 25parallels a portion of the output circuit.

For high frequency operation, such as for the broadcast range and forthe higher frequencies above that range, the use of a bypass condenseraround phones 23 is not essential. It is found in practice that theimpedance of the translating device 23, such as telephone receivers orthe primary of an audio frequency transformer, varies greatly to radiofrequency currents; that is, owing to the distributed capacities of sucha device certain radio frequencies are allowed to pass through thisdevice. In that case these currents pass from plate 13 through coil 22and back to --fila1nent 12 through the capacity ;of the translatingdevice 23. This energy .passing through coil 22 induces radio frequency7 potentials across coil :25 which feeds back to coil 19. The amount offeed-back is .de-

pendent on the value of 'capacity 26.

In the case where the impedance of the translating devlce .23 is such asto exclude these radio frequencies they are forcedtotake the :paththrough plate 1 3, coil 22, coil 25 and capacity 26; the amount-offeed-back to coil 19 again depending'on the value of capacity.o-fcondenser26. p I

I found that for the ordinary broadcast range and when the usual storagebattery .tubes are used, coil 22 can be from three to five turnsand coil25 from eight to fifteen or more turns and condenser 26 can be about.00025 inicrofarads. These values of .course depend onthe degreeoficoupli-ng between coils 19,, .22, 25. :F or the smaller dry celltubes, the member of turns can be increased. Due to the step-up.eifectof coils22.and25,.andthe precise-control of regeneration bycondenser in the series; this is merely .an ralternative, the

theory of operation being thesame as ifibut onetube were used. Coil 18is coupled asbefore to coil 19., formingatunablecircuit with condenser21. Tube 2'? affected by this circuit is a radio frequency amplifier,the input of which is affected by coils .28 and .29., associated withthe output circuit of succeeding tube 30, shown in this instance as adetector. The output circuit of tube 27 includes @011 33 thatserves to:affect the tunable circuit 34 in the input circuit of tube 30. Theoutput circuit of tube .30 includes plate 35,, 003L129,

translating device 36 such as a loud speaker bridged by a small capacity37, a portion of the common B battery 38, and filament 39.

In this form, I also take care of instability that may resultin amulti-stage amplifier at the high frequency end of the range. lVhensignals at the high frequency end of the range are tuned in, thecircuits are unstable, and undesired oscillations may result. Under suchcircumstances, positive regeneration is a detriment, for it assists theoscillations. lVith the present scheme, the coil 29 opposes the coil 19.Therefore a reversed feed-back effect is secured. Condenser 40 incircuit with coil 28 however can be adjusted just up to the point whereinstability begins. When signals at lower frequencies come in, thecapacity is increased so that the positive regenerative effect of coil28 becomes larger and larger, and finally it overcomes the reversedeffected of coil 29 and the net result is positive regeneration at thelower frequencies. Thus it is seen that the system can be varied througha range between negative regeneration to positive regeneration merely bygradually increasing the capacity of condenser 40. It is seen that thenumber of turns of coil 28 should be greater than that of coil 29 sothat a step-up transformer efiect can be secured between these twocoils, whereby the negative regeneration of coil 29 can be finallyovercome for the lower frequency signals.

It is to be noted that coil 29 is in series both with the plate 35 anddevice 36 and therefore serves as a direct medium for transfer of energybetween the output of tube 30 and the input of tube 27 The circuit forcoil 28 parallels a portion of this branch of the output circuit, saidportion including coil 29 and device 36 so that this coil circuit canalso affect the input of tube 27. The variation in the effect of coil 28is secured as before, by varying condenser 40. In this instance, coils28 and 29 can be wound in the same direction to secure an opposedefiect, because the position of the terminals of coil 29 is reversedwith respect to plate 35 and translating device 36, as compared with theposition of the terminals of coil 22 with respect to plate 13 and device23. In this way, the coils 28 and 29 are opposed, instead of beingcumulative, as are coils 22 and 25 in Fig. 1.

In the form illustrated in Fig. 1, it is possible to make condenser 26fixed at a medium value, whereby the regeneration is at an efficientpoint for the entire range without the necessity of adjustment; thevalue of capacity 26 being then low enough to prevent oscillation of thedetector circuits, and the amount of regeneration of the system is controlled in one or more of the other stages. It is also of coursepossible, when oscillations are desired to be generated, to fix thevalue of capacity 26 by a fixed condenser so as to throw the system intooscillation for the entire range.

I claim:

1. In combination, one or more electronic emission devices having aplurality of electrodes, an input circuit connected to some of theelectrodes, an output circuit connected to some of the electrodes, andmeans for feeding back energy from the output circuit to the inputcircuit, comprising a pair of inductances and a capacity connected inseries and between the input and output circuits, the inductances beinginductively coupled to the input circuit, said output circuit includinga translating device in series with one only of said inductances..

2. An inductively coupled regenerative system for one or more electronemission devices, having an input and an output circuit, characterizedby'the provision of a pair of cumulatively wound coils in an outputcircuit, coupled to an input circuit, one of said coils being moreclosely coupled than the other, and means for varying the efiect of isaid more closely coupled coil on the input circuit.

3. An inductively coupled regenerative system for one or more electronemission devices, having an input and an'output circuit, characterizedby the provision of a pair of coils in an output circuit, both beingcoupled to the input circuit, a translating device in series with one ofsaid coils, and the other of said coils being in parallel with thetranslating device, and variable impedance means in series with saidother coil and forming a circuit therewith the impedance of which isvariable.

4. The combinationas set forth in claim 3, in which said other coil isplaced closer to the input circuit than the said one of the coils, andhas a larger number of turns, whereby a stepup effect is obtainedbetween the two coils by energy passing through that coil which is inseries with the translating device.

5. The combination as set forth in claim 3, in which the variableimpedance means is a variable condenser.

In testimony whereof I have hereunto set my hand.

PERCIVAL J. TOWNSEND.

